I would like additional help on a answer to this question, Evaluating a math expression given in string form. The user @Boann answered the question with a very interesting algorithm that he also points out can be altered to accept variables. I've managed to alter it and get it to work, but dont know how he separates the compilation and evaluation. Here's my code:
import java.util.HashMap;
import java.util.Map;
public class EvaluateExpressionWithVariabels {
@FunctionalInterface
interface Expression {
double eval();
}
public static void main(String[] args){
Map<String,Double> variables = new HashMap<>();
for (double x = 100; x <= +120; x++) {
variables.put("x", x);
System.out.println(x + " => " + eval("x+(sqrt(x))",variables).eval());
}
}
public static Expression eval(final String str,Map<String,Double> variables) {
return new Object() {
int pos = -1, ch;
//if check pos+1 is smaller than string length ch is char at new pos
void nextChar() {
ch = (++pos < str.length()) ? str.charAt(pos) : -1;
}
//skips 'spaces' and if current char is what was searched, if true move to next char return true
//else return false
boolean eat(int charToEat) {
while (ch == ' ') nextChar();
if (ch == charToEat) {
nextChar();
return true;
}
return false;
}
Expression parse() {
nextChar();
Expression x = parseExpression();
if (pos < str.length()) throw new RuntimeException("Unexpected: " + (char)ch);
return x;
}
// Grammar:
// expression = term | expression `+` term | expression `-` term
// term = factor | term `*` factor | term `/` factor
// factor = `+` factor | `-` factor | `(` expression `)`
// | number | functionName factor | factor `^` factor
Expression parseExpression() {
Expression x = parseTerm();
for (;;) {
if (eat('+')) { // addition
Expression a = x, b = parseTerm();
x = (() -> a.eval() + b.eval());
} else if (eat('-')) { // subtraction
Expression a = x, b = parseTerm();
x = (() -> a.eval() - b.eval());
} else {
return x;
}
}
}
Expression parseTerm() {
Expression x = parseFactor();
for (;;) {
if (eat('*')){
Expression a = x, b = parseFactor(); // multiplication
x = (() -> a.eval() * b.eval());
}
else if(eat('/')){
Expression a = x, b = parseFactor(); // division
x = (() -> a.eval() / b.eval());
}
else return x;
}
}
Expression parseFactor() {
if (eat('+')) return parseFactor(); // unary plus
if (eat('-')){
Expression b = parseFactor(); // unary minus
return (() -> -1 * b.eval());
}
Expression x;
int startPos = this.pos;
if (eat('(')) { // parentheses
x = parseExpression();
eat(')');
} else if ((ch >= '0' && ch <= '9') || ch == '.') { // numbers
while ((ch >= '0' && ch <= '9') || ch == '.'){
nextChar();
}
double xx = Double.parseDouble(str.substring(startPos, this.pos));
x = () -> xx;
} else if (ch >= 'a' && ch <= 'z') { // functions
while (ch >= 'a' && ch <= 'z') nextChar();
String func = str.substring(startPos, this.pos);
if ( variables.containsKey(func)){
x = () -> variables.get(func);
}else{
double xx = parseFactor().eval();
if (func.equals("sqrt")) x = () -> Math.sqrt(xx);
else if (func.equals("sin")) x = () -> Math.sin(Math.toRadians(xx));
else if (func.equals("cos")) x = () -> Math.cos(Math.toRadians(xx));
else if (func.equals("tan")) x = () -> Math.tan(Math.toRadians(xx));
else throw new RuntimeException("Unknown function: " + func);
}
} else {
throw new RuntimeException("Unexpected: " + (char)ch);
}
if (eat('^')){
x = () -> {
double d = parseFactor().eval();
return Math.pow(d,d); // exponentiation
};
}
return x;
}
}.parse();
}
}
If you take a look at his answer his main
public static void main(String[] args) {
Map<String,Double> variables = new HashMap<>();
Expression exp = parse("x^2 - x + 2", variables);
for (double x = -20; x <= +20; x++) {
variables.put("x", x);
System.out.println(x + " => " + exp.eval());
}
}
He calls function parse on this line Expression exp = parse("x^2 - x + 2", variables); to compile the expression once and uses the for to evaluate it multiple times with a unique x values. What does the parse function refer to.
ps: I have commented on the users question with no reply.
